The principle of scanning objects with high energy radiation such as x-rays or gamma-rays, particularly to generate image information in the form of a transmission radiograph, is widely employed for example in the security industry, but might also be employed in other areas, for example, without limitation, medical imaging, imaging for quality control purposes or the purposes of determining the integrity of the structure, or the like.
Security scanning is particularly directed at the identification of undesirable materials or objects, and especially explosives or weapons, in airline baggage. Airport security must in particular ensure that no explosive material is allowed on board any aircraft. Various strategies are employed in order to achieve this goal. However one of the most important is the transmission screening of hold baggage using x-ray machines that have automated explosive detection capability. Radiation-based systems such as x-ray systems also find wide application in medical radiography and in some quality and process control applications.
Analysis based on transmitted radiation, whether for transmission radiography or otherwise, relies on the same general principle. The thicker or more dense an object is then the more it will be likely to attenuate an incident beam. By use of suitable detectors and a suitable source, transmitted intensity data can be used to derive information, for example in the form of radiographs, of an item under screening based on the absorption of an object or set of objects can be generated. X-Ray absorption has also been used for some time as the basis for screening objects to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space.
This approach may be limited in that it tends to give limited information about the material content. At its simplest, all that is being measured is transmissivity. The detector merely collects amplitude information.
However, it is known that the absorption properties of any material can vary with energy, and that the amount by which the absorption properties vary depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately differentiating, at least to some degree, between low- and high-energy bands from the full spectrum of an x-ray source. U.S. Pat. No. 4,247,774 represents a general reference to the use of a dual-energy detector system in relation to computer assisted topography in a medical, imaging application.
A dual energy system confers only limited information about composition. Recent development of detectors that can resolve spectroscopic information about transmitted x-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of images across these bands to generate multispectral images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images.
A particular problem presented by practical objects is that a practical object is rarely homogeneous, but is often composed of a number of constituent components each of which can be expected to have its own characteristic absorption properties. A conventional transmission radiograph, for example built up of data in one or two dimensions in an x, y plane generally perpendicular to a radiation beam direction is thus limited in that since it delivers information about transmitted intensity only it is only possible to show the cumulative absorption effect in the ray path in such a radiograph. To distinguish between such component materials, and to obtain an indication of the relative contribution of, and for example the relative thickness of, such component materials it has traditionally been necessary to employ more complex scanning geometries, for example so as to generate multiple ray paths through an object in multiple directions.